Annulate Animals to their Economy, 237 



vertebrates. The marine annulates farther resemble fishes 

 in breathing by gills ; a most beautiful and wonderful con- 

 trivance, which enables them to extract from water a suf- 

 ficient quantity of air to aerate the blood, and thus pre- 

 cludes the necessity, so obviously attendant on all other 

 modes of respiration, namely, a constantly repeated ascent to 

 the surface for fresh supplies of atmospheric air. In these 

 animals, consequently, the blood must ascend to the gills, in 

 order to be brought into contact with the air so produced ; 

 but in the hexapod or true insects, which constitute by far 

 the greater portion of the annulate kingdom, the respiration 

 and aeration of the blood are managed on an entirely different 

 principle. 



The circulation of blood in hexapod insects we have 

 already examined ; let us now turn to the circulation of air. 

 This is not confined to a single region, as the gills in fishes, 

 or the lungs in man ; but seems to be almost universal : it 

 consists of an infinity of tubes ramifying through the body in 

 every direction, from one end to the other. These air-tubes, 

 termed tracheae, have generally nine nearly equidistant ex- 

 ternal openings, termed spiracula, on each side of an insect. 

 From each spiraculum a main trachea seems to enter the 

 body, and immediately divides into two, three, four, or five 

 minor tubes, one of which usually unites with the next spira- 

 culum ; thus forming a principal trachea, which extends the 

 whole length of the body, on each side of the great dorsal 

 channel for the blood already spoken of. From the main 

 spiracular tracheae minor transverse tracheae frequently arise, 

 and, passing above and below the great dorsal channel, unite 

 with similar minor tracheae, twigging off from the opposite 

 side ; and thus the whole system of tubes becomes connected, 

 and acts by a common impulse. 



To a casual observer, it may seem, perhaps, somewhat 

 strange that a system of respiratory organs so apparently 

 complicated should have been given to a race of beings which 

 we consider of so very little importance in the universe : but 

 an examination, however cursory, will show that here, as in 

 every other of the Almighty's works, there is nothing super- 

 fluous ; that there has been no labour in vain. 



Respiration and circulation are so intimately connected, 

 that the operation of each depends on the operation of the 

 other. Now, in insects the blood is cold ; and for this excel- 

 lent reason, that, in so small a bulk as that of an insect's body, 

 so large a portion of the blood is exposed to the air, that a 

 heat superior to that of the atmosphere could not be main- 

 tained. This being the case, circulation of the simplest pos- 



